DE3788388T2 - Process for producing a REFET or a CHEMFET, and their production. - Google Patents

Abstract

The invention relates to a process for manufacturing a REFET and/or CHEMFET, comprising: a) covalent bonding of a hydrophilic polymer layer to an isolator layer applied to a semiconductor material; b) the absorption of water or a watery solution into said hydrophilic polymer layer; and c) the binding of a hydrophobic polymer layer to the water holding hydrophilic polymer layer. It is advantageous that this water holding hydrophilic polymer layer also contains an electrolyte and/or a buffer. Preferably an ion to be measured with the CHEMFET also forms part of the electrolyte.

Description

The present invention relates to the manufacture of a REFET, whereby an ISFET is coated with an inert hydrophobic layer, or a CHEMFET, whereby an ISFET is modified with a hydrophobic layer containing a receptor molecule for a chemical compound to be determined, for example a cation, anion, proton, urea, sugar, protein, antibody, antigen. ISFET (ion-sensitive field effect transistor) is understood to mean a semiconductor material that is provided with an insulating layer. The ISFETS are manufactured in accordance with the usual MOS technology. The insulating layer is thermally deposited for 15 minutes at 1150°C in an oxygen atmosphere. The layer thickness is up to approx. 700 Å.

Compared with conventional ion-selective electrodes, REFET and CHEMFET have a number of advantages:

1) they are small and therefore suitable for biomedical applications ;

2) they are robust;

3) they can be manufactured using IC technology, which enables the production of a miniature multi-sensor;

4) mass production at low cost is possible without any problem;

5) as a result of the low output impedance, the sensitivity to external interference signals is lower; and

6) They have a better signal-to-noise ratio.

The insulator layer (commonly referred to as lattice oxide layer) applied to the semiconductor material can, for example, consist either of layers of SiO₂, Al₂O₃, Ta₂O₅, TiO₂, ZrO₂ and/or Si₃N₄ (no oxide!) or of a layered structure made of different materials.

The outer surface of the insulator layer contains chemically reactive, particularly protonic groups (silanol groups in the case of SiO2 and amino groups in the case of Si3N4). These groups ensure that the chemically unmodified ISFET shows a response to changes in proton concentration. It is therefore desirable for measuring other chemical compounds to introduce steps to suppress or eliminate proton sensitivity. These steps can consist either of allowing the reactive groups to react, with the result that the reactivity decreases, of covering the insulator layer with an inert hydrophobic layer, or of binding an inert hydrophobic polymer to the insulator layer. The last possibility is described in detail in the international patent application WO 85/04480 in the name of the applicant, the Stichting Centrum voor Micro-Electronica Twente of Enschede.

If the hydrophobic polymer contains ionophores, a specific ion sensitivity and ion specificity occur, which are specifically dependent on the ionophores. Such an ISFET is called a CHEMFET. An H+-insensitive ISFET is called a REFET.

After extensive investigation, it has been found that the characteristics of a sensor with such a REFET and/or CHEMFET as an essential connection can be significantly improved in terms of background noise, drift and hysteresis. This is achieved according to the invention when a method for producing a FET comprises: a) covalently bonding a hydrophilic polymer layer to an insulator layer applied to a semiconductor material; b) absorbing water or an aqueous solution into the hydrophilic polymer layer; and c) bonding a hydrophobic polymer layer to the water-storing hydrophilic polymer layer. Reproducibility is also improved during production.

It was found during this investigation that many of the problems associated with the characteristics described above could be traced back to unverifiable variations in the chemical structure at the interface between the insulator layer and the hydrophobic polymer layer deposited on top of it. These variations were associated with the manufacturing process conventionally used.

The application of the water-storing, hydrophilic polymer layer between the insulator layer and the hydrophobic polymer layer leads to thermodynamically and chemically better defined boundary layers between the insulator layer and the hydrophilic polymer layer on the one hand and between the hydrophilic polymer layer and the hydrophobic polymer layer on the other.

The hydrophilic polymer layer should be covalently bonded to the insulator layer, otherwise the durability will only last for several hours, which is unsuitable for practical use.

The hydrophilic polymer layer preferably also contains an electrolyte so that the potential drop across the interface between the hydrophilic polymer layer and the hydrophobic polymer layer is limited and is essentially independent of the composition of the solution to be determined. In the case of a CHEMFET, the ion to be determined with the CHEMFET preferably forms part of the electrolyte.

It is further recommended that the hydrophilic polymer layer also contains a buffer. In this way, the pH in the hydrophilic polymer layer is kept substantially constant, even when a flow of H+ ions or a flow of CO2 occurs through the hydrophobic polymer layer. A particularly preferred CHEMFET or REFET is obtained when the hydrophilic polymer layer contains an electrolyte as well as a buffer.

The hydrophilic polymer layer has a thickness of 0.0050- 200 µm, and preferably 0.1-100 µm, and more preferably 1-50 µm.

Various possibilities are possible for the preparation of a hydrophilic polymer layer that is covalently bonded to the insulator layer.

For the covalent bonding, it is possible, on the one hand, to use a bifunctional organosilicone compound containing, on the one hand, a group that can be covalently bonded to the insulator layer and, on the other hand, a group to which either monomers that are to be made to react to form polymers or polymerized material can be bonded.

Organosilylation reagents that are used are mentioned in (P. Bergveld, N.F. de Kooy, Ned. Tijdschr. v. Naturkunde (Dutch Journal of Physics), A46, 22-25 (1980)) The organosilicone compounds I and II from Table 1 are particularly suitable.

After the bifunctional organosilicone compound has been covalently bonded to the insulator layer, the hydrophilic polymer layer can be formed by polymerization of monomers that are matched to the covalently bonded bifunctional organosilicone compound. Photochemically polymerizable monomers are used, for example, to enable IC compatible mass production. In the case of compounds I and II, the hydrophilic polymer layer can be formed by polymerization and comprises

a) Monomers with the structural formulas:

where R³ is H, alkyl, aryl, halogen

R4: -H

-R⁵-OH

R⁵-NH₂

R5-SH

R⁵-N(CH₃)₂

R&sup5;-sugar

R5-peptide

RS: Alkyl, Aryl

B: OH, NH2,

b) copolymers with additional monomers having the structural formula III, in which R⁴ now represents a C₁-C₃₀ alkyl group,

c) mixtures of (co)polymers with a hydrogel such as gelatin, agar-agar, heparin and polyvinylpyrrolidone,

d) mixtures of monomers III and IV,

e) mixtures of polymers and/or copolymers of monomers III and IV,

f) mixtures of the (co)polymers of monomers III and/or IV with the copolymers according to point b,

g) Mixtures of the polymers and/or copolymers listed under points a-f.

Copolymers of the above-mentioned monomers can also be formed with vinylsilanes (Table 1). These materials can be covalently bonded directly to the insulator layer.

In this way, the hydrophilic polymer layer is applied to the insulator layer.

Water or an aqueous solution is then allowed to penetrate into the hydrophilic layer by immersion or by adding a necessary amount of water.

Finally, the hydrophobic polymer layer is applied to the hydrophilic polymer layer, which now contains water. The bonding can be done physically or covalently (chemically).

In the case of a covalent bond, a cross-linking agent with the structural formula can be used:

where R³ has the meaning given above and Z = halogen, alkoxy, phenoxy or hydroxy.

The hydrophobic polymer layer is also formed by polymerization of monomers with vinyl groups or by reaction with "living" polymers.

The use of a cross-linking agent is necessary when, when covalent bonding is required, it is not possible to make the hydrophobic layer react covalently with existing functional groups of the hydrophilic polymer layer.

In the case of the crosslinking agent mentioned above, use can be made of monomers containing vinyls referred to in WO 85/04480 and, in addition, styrene, divinylbenzene, acrylonitrile, vinyl acetate, methyl (meth)acrylate, butyl (meth)acrylate, vinylidene cyanide, chlorostyrene and, for example, chloromethylstyrene.

It is of course also possible to use silylating agents as cross-linking agents. (See: Deschler, U., Kleinschmit, P., Panster, P.; Angew. Chemie, 98, 237-253, (1986)). In addition, it is also possible to make use of polymers that are provided with reactive silane monomers.

In the case of applying a polymer layer made of polyurethanes or polycarbamates or other polymers which are reactive with respect to the hydrophilic layer, it is not necessary to make use of a cross-linking agent.

It is also possible to physically bond the hydrophobic polymer layer to the hydrophilic polymer layer. This physical bond is achieved using the polymers described above, but without the cross-linking agent. Plasticized PVC, siloxane rubber and polybutadiene can also be used as polymers.

Finally, it is also possible to use so-called living polymers.

If no receptor is incorporated into the inert hydrophobic polymer layer, a sensor in which such a FET is incorporated will act as a so-called REFET. This REFET gives a non-ion-specific response to the variation in the total ion concentration. If the ionic strength in the solution under investigation is constant, it therefore acts as a pseudo-reference electrode.

When a receptor is incorporated into the hydrophobic polymer layer, a sensor equipped with it functions as a CHEMFET. The receptor can be covalently or physically bound into the hydrophobic polymer layer, or incorporated into it during polymerization.

The receptor can also be an ionophore such as an antibiotic (e.g. valinomycin), a crown ether, cryptate, podand, memispherand, cryptohemispherand or spherand.

A H+-sensitive CHEMFET is prepared by incorporating a compound with acidic or basic groups, for example tridodecylamine as an ionophore, into the inert hydrophobic polymer layer.

As a result of the selection of a specific receptor and its incorporation into the hydrophobic polymer layer, it is possible to selectively determine the presence and (thermodynamic) activity of chemical and especially biochemical compounds such as ions, proteins, substrates, antibodies, antigens, hormones, gases, glucose, urea, etc. in solution.

The fact is emphasized that, of course, the thermodynamic activity of both cations and anions can be determined.

The present invention will be illustrated with reference to a number of inventive examples and reference examples, although the invention is in no way limited thereto.

example 1

A wafer of a semiconductor material provided with an insulator layer of SiO₂ is placed in a mixture of methacryloxypropyltrimethoxysilane (Alfa, US) and toluene, which is boiled under reflux conditions, for two hours. After washing with methyl ethyl ketone, the wafer is dried for one hour at 80°C. Hydroxyethyl methacrylate (HEMA; Janssen, Belgium) is then applied according to the so-called dipping method. The polymerization and the covalent bonding to the insulator layer are carried out photochemically. The polymerization reactions are carried out using the Photoinitiators 2,2-dimethoxy-2-phenylacetophenone (4 wt.%; Janssen, Belgium) present in the mixture to be polymerized are initiated by irradiating the mixture with an ultraviolet light source (λmax = 360 nm) for 2-10 minutes under a N₂ atmosphere.

The thickness of the hydrophilic polymer layer is 15 μm and is determined in accordance with ellipsometry.

The covalently bound hydrophilic polymer layer is swollen for 30 minutes at room temperature with a solution of 0.1 M KCl + 0.025 M KH₂HPO₄ in water.

A polybutadiene solution (mol. wt. 4300, 99% unsaturated, 25% vinyl and 40% trans-1,4, available from Janssen, Belgium) in tetrahydrofuran is then applied to the water-storing, hydrophilic polymer layer and then photochemically polymerized. The thickness of the hydrophobic polymer layer, which is covalently bonded to the hydrophilic, water-storing polymer layer, is 10 µm.

Example 2

In a similar manner to that described in Example 1, a hydrophilic polymer layer is applied to the insulator layer. The outer side of the hydrophilic polymer layer is then functionalized by reaction with methacryloxychloride. In this way, a portion of the hydroxyethyl groups are converted to methacryloxy groups and the hydrophilic polymer layer is swollen in the aqueous, buffered KCl solution as described in Example 1.

Then, a mixture of ACE (C9H19C(O)OCH2CH(OH)CH2OC(O)CH=CH2) and Epocryl (p.R-C6H4-C(CH3)2-C6H4p.R; R is CH2=C(CH3)C(O)OCH2CH(OH)CH2O-) (20:80 w/w; available from Shell, Netherlands) in chloroform is applied to the hydrophilic, water-storing polymer layer, which is then photochemically polymerized. The layer thickness of the hydrophobic polymer layer is 15 µm and it is cross-linked with the applied functional methacryloxy groups.

Example 3

In the same manner as in Example 1, a hydrophilic polymer layer is covalently applied to the insulator layer and swollen in the aqueous, buffered KCl solution as described in Example 1.

A solution of polyvinyl chloride (PVC, available from Fluka, Switzerland, purum; for ion-selective electrodes 30 wt.%), di-n-butyl phthalate (67 wt.%) and valinomycin (available from Fluka, Switzerland, purum pro analysis; 3 wt.%) in tetrahydrofuran is then applied. The resulting hydrophobic polymer layer has a thickness of 10 µm.

Example 4 (reference example)

A wafer of a semiconductor material provided with an insulator layer of SiO2 is placed in a mixture of methacryloxypropyltrimethoxysilane and toluene boiled under reflux conditions for two hours. After thorough washing with methyl ethyl ketone, the wafer is dried at 80°C for one hour. This wafer is then placed in a solution of polybutadiene (MW 3400, 99% unsaturated, 25% vinyl and 40% trans-1,4, available from Janssen, Belgium) in tetrahydrofuran. Photochemical polymerization is then carried out. The thickness of the hydrophobic polymer layer covalently deposited directly on the insulator layer is 10 µm.

Example 5 (reference example)

The silylation process is carried out in the same way as in Example 4. A mixture of ACE and Epocryl in chloroform is then applied and photochemically polymerized.

The thickness of the hydrophobic polymer layer, which is covalently bonded directly to the insulator layer, is 15 µm.

Example 6 (reference example)

After carrying out the silylation process according to Example 4, a physically bound hydrophobic Polymer layer is bonded directly to a wafer of a semiconductor material provided with an insulating layer of SiO₂. For this purpose, a solution of polyvinyl chloride (30 wt.%), di-n-butyl phthalate (67 wt.%) and valinomycin (3 wt.%) in tetrahydrofuran is added. The layer thickness of this physically bonded hydrophobic polymer layer, which is provided with a receptor, is 10 µm.

Example 7 (reference example)

A layer of gelatin from an approximately 3 wt.% gelatin solution in water is physically bonded to a wafer of a semiconductor material provided with an insulating layer of SiO2. The thickness of the gelatin layer is 25 µm.

A mixture of ACE and Epocryl (40:60 w/w) in chloroform is then applied to the gelatin layer and photochemically polymerized as described above. The layer thickness of the hydrophobic polymer layer is 15 µm.

Example 8

A mixture of three parts by weight of methacryloxypropyltrimethoxysilane and 7 parts by weight of hydroxyethyl methacrylate (HEMA) in toluene is photochemically polymerized. The toluene is distilled off and the product is dissolved in acetone. Part of the polymer solution prepared in acetone is applied to the plate of semiconductor material to be treated, which is provided with an insulating layer of SiO₂. After the acetone has evaporated, the semiconductor material is placed in an oven at 80°C for 24 hours. The hydrophilic polymer layer thus covalently bonded has a thickness of 20 µm. The aqueous, buffered KCl solution is then absorbed by this polymer as described in Example 1. A mixture of ACE and Epocryl is then applied to this swollen polymer layer and polymerized as described in Example 2. The layer thickness is 15 um.

Example 9

A wafer of a semiconductor material provided with an insulating layer of SiO2 is silylated with methacryloxypropyltrimethoxysilane as described in Example 1. A mixture of 50 parts by weight of hydroxyethyl methacrylate and poly-N-vinylpyrrolidone (50 parts by weight; molecular weight = 360,000; Janssen, Belgium) is then applied in accordance with the spin coating process. Polymerization and covalent bonding to the insulating layer is achieved photochemically. The thickness of the hydrophilic polymer layer is 25 µm. The buffered aqueous KCl solution is then absorbed by this polymer as described in Example 1. A hydrophobic polymer layer of PVC, di-n-butyl phthalate and valinomycin is then applied as in Example 3. The layer formed has a thickness of 10 µm.

Example 10

In the same way as in Example 1, a hydrophilic polymer layer is applied to the insulator layer and swollen in the aqueous, buffered KCl solution as described in Example 1. A mixture of Silopren K 1000 (85% by weight), crosslinking agent KAI (12% by weight, both from Bayer AG, Leverkusen, Germany) and valinomycin (3% by weight) is then applied to the hydrophilic, water-storing polymer layer and crosslinked. The layer thickness is 15

Example 11 (reference example)

The silylation process is carried out in the same way as in Example 4. A mixture of Silopren K 1000 (85% by weight), crosslinking agent KA-1 (12% by weight) and valinomycin (3% by weight) is then applied and crosslinked. The layer thickness is 15 µm.

Example 12

A wafer of semiconductor material provided with an insulating layer of SiO₂ is then Example 1 described with methacryloxypropyltrimethoxysilane. A mixture of hydroxyethyl methacrylate (60 parts by weight) and N-vinyl-2-pyrrolidone (40 parts by weight, Janssen, Belgium) is then applied using the dipping method. Polymerization and covalent bonding to the insulating layer is carried out photochemically. The thickness of the hydrophilic polymer layer is 20 µm. The aqueous, buffered KCl solution is then absorbed by this polymer as described in Example 1. A mixture of ACE and Epocryl is then applied to this swollen polymer layer and photochemically polymerized as in Example 2. The layer thickness is 15 µm.

Example 13

A wafer of semiconductor material provided with an insulating layer of SiO2 is silylated with methacryloxypropyltrimethoxysilane as described in Example 1. N,N-dimethylaminoethyl methacrylate (DMAEMA, Janssen, Belgium) is then applied in accordance with the dipping process. Polymerization and covalent bonding are carried out photochemically. The thickness of the hydrophilic polymer layer is 25 µm. An aqueous buffered KCl solution is then absorbed by this polymer as described in Example 1. A mixture of ACE and Epocryl is then applied to this swollen polymer layer and photochemically polymerized as in Example 2. The layer thickness is 15 µm.

Example 14

As in Example 1, but here only water is absorbed by the hydrophilic polymer.

Example 15

As in Example 1, but here 0.10 M KCl in water is absorbed by the hydrophilic polymer.

Example 16

A wafer of a semiconductor material provided with an insulating layer of SiO2 is silylated with methacryloxypropyltrimethoxysilane as described in Example 1. A hydrophilic polymer layer is then applied, which is then functionalized by reaction with methacryloxychloride, after which the aqueous buffered solution is absorbed as described in Example 1. A mixture of polybutadiene (for details see Example 1) and 4-vinylbenzo-18-crown-6 (5% by weight) is then applied to the hydrophilic polymer layer and photochemically polymerized. The layer thickness is 10 µm.

Determination of the characteristics of the fabricated REFET and CHEMFET

The modified REFET and CHEMFET were measured in accordance with the procedure described by A. van den Berg, P. Bergveld, D.N. Reinhoudt and E.J.R. sudhölter, Sensors and Actuators 8 129-148 (1985).

The results are shown in Table 2.

In addition, the reproducibility of the fabricated REFETs and CHEMFETs in mass production is determined and is shown in Table 3.

A sensor equipped with a REFET such as that of Example 7 showed a continuous increase in response during measurement at pH 7. After exposure to this solution for five minutes, a pH response consistent with an unmodified ISFET was observed.

It was found that a sensor with a REFET as prepared in Example 2 still showed the original, uniform pH response even after being stored at pH 7 for one month. Table 1 Bifunctional organosilicone compounds for covalently bonding the hydrophilic polymer layer to the insulator layer Organosilicone compound Structural formula Vinylsilane

Remarks:

* R, R1 : Alkyl, Aryl

X : Halogen, Carboxylate, Amino, Oxime

Y : H, alkyl or aryl

A:

a+b+c=3

a = 1, 2 or 3

p ≥ 2 Table 2 Measured characteristics of REFETs and CHEMFETs Example Noise Drift Hysteresis

Remarks:

1) Drift measured after two hours of stabilization in the solution at pH 7 (start of measurement after stabilization)

2) Hysteresis, measured after a pH measurement from pH 2 to pH 10 (measurement time 10 minutes) followed by a pH measurement from pH 10 to pH 2 (measurement time 10 minutes). The �Lambda;mV value at pH 2 is given.

3) This involves a titration with KCl from 10⁻⁴ M to 10⁻¹ M, followed by washing and immersion in a solution of 10⁻⁴ M. ΛmV is given for 10⁻⁴ M.

4) Not stable

Table 3 Reproducibility of the procedure

Example Percentage *

1 80

2 90

3 85

4 30

5 10

6 50

7 not durable

8 85

9 90

10 80

11 40

12 80

13 85

14 80

15 80

16 75

Annotation:

* Reproducibility is expressed as the ratio of the number of modified FETs with noise equal to or better than the value given in Table 2 and the total number of FETs fabricated in this way.

Claims (15)

1. A method for producing a REFET and/or CHEMFET, comprising: a) covalently bonding a hydrophilic polymer layer to an insulating layer applied to a semiconductor material; b) the absorption of water or an aqueous solution into the hydrophilic polymer layer; and c) bonding a hydrophobic polymer layer to the water-storing, hydrophilic polymer layer. 2. A method as claimed in claim 1, wherein the water-storing, hydrophilic polymer layer also contains an electrolyte. 3. A method as claimed in claim 1 or 2, wherein the water-retaining hydrophilic polymer layer also contains a buffer. 4. A method as claimed in claims 1 to 3, wherein the water-storing hydrophilic polymer layer has a thickness of 0.0050-200 µm, and preferably of 0.1-100 µm, and more preferably of 1-50 µm. 5. A method as claimed in claims 1 to 4, wherein a receptor for a chemical compound to be determined is incorporated into the hydrophobic polymer layer. 6. A method as claimed in claim 5 and at least claim 2, wherein an ion to be determined with the CHEMFET also forms part of the electrolyte. 7. A method as claimed in claims 1 to 6, wherein the hydrophilic polymer layer is covalently bonded to the insulator layer via a bifunctional organosilicone compound, on the one hand via a group which can be covalently bonded to the insulator layer and on the other hand via a group which can be covalently bonded to the hydrophilic polymer layer, 8. A method as claimed in claim 7, wherein the bifunctional organosilicone compound compounds having the following structural formulas: where R, R' : Alkyl, Aryl X : Halogen, Carboxylate, Amino, Oxime Y : H, alkyl or aryl O A: a+b+c=3 a = 1, 2 or 3 and p ≥ 2 9. A method as claimed in claim 7 or 8, wherein the hydrophilic polymer layer is formed by polymerization and comprises a) Monomers with the structural formulas: where R³: H, alkyl, aryl, halogen R⁴: -H -R⁵-OH R⁵-NH₂ R5-SH R⁵-N(CH₃)₂ R³-sugar R5 peptide R5: Alkyl, Aryl B : OH, NH₂, b) copolymers with additional monomers having the structural formula III, where R⁴ is now, for example, a C₁-C₃O alkyl group, c) mixtures of (co)polymers with a hydrogel such as gelatin, agar-agar, heparin and polyvinylpyrrolidone, d) copolymers of monomers III and IV, e) mixtures of polymers and/or copolymers of monomers III and IV, f) mixtures of (co)polymers of monomers III and/or IV with the copolymers according to point b, g) Mixtures of the polymers and/or copolymers mentioned under points a - f. 10. A method as claimed in claims 1 to 9, wherein the hydrophobic polymer layer is physically bonded to the hydrophilic, water-storing polymer layer. 11. A method as claimed in claims 1 to 9, wherein the hydrophobic polymer layer is covalently bonded to the hydrophilic, water-storing polymer layer. 12. A method as claimed in claim 11, wherein a crosslinking agent is allowed to react with the hydrophilic polymer layer and the hydrophobic polymer layer is then bonded to the crosslinking agent. 13. A method as claimed in claim 12, wherein the crosslinking agent has the following structural formula: where R³ has the meaning given above, and z = halogen, alkoxy, phenoxy or hydroxy, and the hydrophobic polymer layer is formed by polymerization of monomers with vinyl groups or by reaction with "living" polymers. 14. A method as claimed in claim 6 and, if necessary, in claims 7 to 13, wherein the receptor is covalently or physically bound to the hydrophobic polymer layer or is cross-linked. 15. REFET or CHEMFET, prepared as claimed in any one of claims 1 to 14.